This invention relates to flexible heating cables and particularly to temperature-regulated heating cables useful in the heat tracing of pipes or the like to maintain such pipes at or above a predetermined temperature regardless of ambient conditions.
Most prior art heat tracing cables utilize a pair of insulated power lines helically wrapped along their common length by resistance wire segments each connected across the power lines to provide constant power at a fixed line voltage. Constant power resistance wire heat tracing cables are illustrated by U.S. Pat. Nos. 3,757,086 and 4,037,083.
This in turn means that to assure attainment of a desired minimum temperature throughout such length, the coolest spot must be heated to that temperature whereby other locations are necessarily heated to higher temperatures, thereby wasting power.
Temperature regulation of such a prior art resistance wire heat tracing cable is typically provided by a separate thermostatic control responsive to the temperature of the associated traced pipe. The thermostatic control regulates cable temperature by initiating or interrupting energization of the power lines across which the resistance wire segments are connected.
Such a heat tracing system utilizing separate thermostatic controls and constant power cables energized via the controls possesses obvious disadvantages in the areas of cost, complexity and reliability. In particular, uniformity of temperature along the length of cable controlled by a thermostat cannot be achieved to the extent that ambient conditions vary along such length. Such lengths typically comprise many meters, so that considerable temperature variation may occur.
It is recognized in the art that a heat tracing cable utilizing integral temperature-responsive means for regulating cable energization is highly desirable. Elimination of separate thermostatic controls significantly simplifies the installation and maintenance of a heat tracing system and distribution of control along the cable length achieves uniformity of temperature.
In prior art attempts to provide temperature self-regulating heating cables, self-regulating heaters or control elements have been distributed along the length of the cable, thereby achieving uniformity of temperature along the length. However, this requires that the total longitudinal extent of the control elements comprise a substantial fraction, or even all, of the length of the cable.
One proposal includes the use of a conductive carbon black extrudate heating element. Such a heating cable is disclosed by U.S. Pat. No. 3,858,144. Here, the control element is equal in total length to the cable. While such a cable may in some applications negate the need for a separate thermostatic control, high cost, limited heat tolerance, limited service life, low maximum temperature, and limited power output offset benefits gained by the elimination of a separate thermostat.
Further prior art attempts to provide an acceptable temperature self-regulating cable include the use of discrete self-limiting heating elements distributed in great number along the length of the heat trace cable so that their total longitudinal extent is a substantial fraction of the cable length. A cable of this type is illustrated by U.S. Pat. No. 4,072,848. While overall cable temperature regulation is provided without the need for a separate thermostatic control, the requirement for high-density longitudinal distribution of heating elements is very costly.
This high cost and the limited power output of such heating elements are serious disadvantages.
Still other prior art efforts to overcome the foregoing problems have included the provision of resistance wire which is helically wrapped around the pair of power leads and around temperature-responsive control elements, such as thermistor chips, along the length of the cable, with connections first to one cable and then to the other. Cables of this type are illustrated in FIGS. 3 and 6 of U.S. Pat. No. 4,117,312. However, such cables are difficult to manufacture economically, even though lowered manufacturing costs have been sought to be achieved by various techniques--for example by reliance on pressure contact alone without use of solder or adhesives, as in the pressure contact component of the FIG. 6 construction of U.S. Pat. No. 4,117,312.
In short, how to provide a cable which performs well, where material costs are not unduly high, and whose parts can be efficiently and economically assembled and interconnected on a mass production basis has long presented a problem.
The present invention solves that problem by a novel arrangement of elements that lends itself to efficient and economic assembly and interconnection. According to the present invention, a series of resistive heating segments are first formed by continuously helically wrapping heating wires around a pair of insulated power leads, with the insulation alternately removed from one power lead and then the other at spaced intervals, to provide a series of positionally serial but electrically parallel circuits or resistance heating segments. The wrapping occurs to form a segment prior to the emplacement of a thermistor chip for that segment. After the heating wire is wrapped on the pair of insulated power leads, a chip is placed against the insulated power leads and over the wrapped-on heating wire. The inner face of the chip becomes electrically connected to the portion of the wrapped-on heating wire that underlies the chip. The lead from the outer face of the chip is electrically connected to the wrapped-on heating wire at a location spaced from the first connection. The wrapped-on heating wire is then severed between the two connections so that any current flowing through the heating wire is diverted through the chip to thereby put the chip in series connection with its associated resistive heating segment.